Characterization of self-emulsifying macadamia nut oil fermented by Epidermidibacterium keratini mutant EPI-7-i originated from skin flora as a novel cosmetic ingredient

Microbial bioconversion has emerged as a sustainable strategy to replace petroleum-based surfactants; however, its application in generating self-emulsifying oils for cosmetic formulations remains limited. This study aimed to develop self-emulsifying oils derived from macadamia nut oil (MNO) through microbial lipid remodeling and to evaluate their physicochemical and functional properties. A skin-derived bacterium, Epidermidibacterium keratini mutant (EPI-7-i), was employed to bioconvert MNO for 2 days (MNO-M1) and 5 days (MNO-M2), after which the self-emulsifying oils were recovered as solvent-extracted lipid fractions from the fermentation system, rather than from the whole culture broth. Lipid composition and emulsion characteristics were analyzed using UHPLC–MS/MS and physicochemical stability assessments. Bioconversion induced extensive lipid remodeling, yielding oils enriched in monoacylglycerols (MAG), diacylglycerols (DAG), free fatty acids (FA), and ether-linked glycerides (alkylacylglycerols, DG O). These compositional changes enabled spontaneous formation of oil-in-water (O/W) nanoemulsions without external surfactants. The emulsions exhibited nanoscale droplet sizes (~ 260 nm), high absolute ζ-potential values (> 45 mV), and stability against thermal and pH stress over 28 days. Despite their high unsaturated lipid content, bioconverted oil emulsions displayed enhanced oxidative stability compared with emulsions prepared from enzymatically modified oil (MNO-E), which was attributed to the formation of microbially derived antioxidant metabolites, including ether lipids structurally analogous to plasmalogens. These results demonstrate that microbial bioconversion enables the integration of emulsification and antioxidative functions within a single oil phase and suggest the potential of solvent-extracted self-emulsifying oils as sustainable, clean-label alternatives for cosmetic nanoemulsion systems.